Oxide superconductor and method for manufacturing the same

ABSTRACT

An oxide superconductor which exhibits an uniform and high critical current density. Further, a method of manufacturing this oxide superconductor, namely, a RE--Ba--Cu--O oxide superconductor (RE is one or more kinds of rare earth elements including Y) by performing a treatment, which includes at least a burning process to be performed in a range of temperatures that are higher than a melting point of a raw material mixture containing a RE-compound raw material, Ba-compound raw material and a Cu-compound raw material, on the raw material mixture. This method further comprises a step of crushing the raw material mixture into particles and establishing the mean particle diameter of one or all of the raw materials as ranging from 50 to 80 μm.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an oxide superconductor which exhibitsexcellent superconducting properties such as a high critical currentdensity, and to a method of manufacturing such an oxide superconductor.Further, the present invention is applicable to, for example, a currentlead, magnetic bearing, a magnetic shielding and to a bulk magnet.

2. Description of the Related Art

Hitherto, there has been known a conventional method of manufacturingsuch a kind of an oxide superconductor (see Japanese Examined PatentPublication No. Hei 7-51463/1996 Official Gazette), by which aRE--Ba--Cu--O oxide superconductor (incidentally, RE is a rare earthelement including Y) is manufactured by performing a treatment on a rawmaterial mixture containing a RE compound, Ba compound and a Cu compound(incidentally, this treatment includes at least a burning (or baking)process to be performed in a range of temperatures that are higher thanthe melting point of such a raw material mixture).

In the case of this conventional manufacturing method, the raw materialmixture, in which the RE compound, the Ba compound and the Cu compoundare mixed in a predetermined mole ratio, is once melted. Thereafter, theraw material mixture is quenched and solidified. Then, the solidifiedraw material mixture is pulverized or crushed into fine powder.Subsequently, such powder is heated again to a temperature of a hightemperature region in which the powdery mixture partially presents aliquid phase. Thereafter, the mixture is gradually cooled. Thus, asuperconducting phase is grown. Furthermore, an oxide superconductorexhibiting a relatively high critical current density can be obtained byperforming annealing process in an oxygen atmosphere.

However, in the case of the aforementioned manufacturing method, thecoagulation and condensing of the raw materials occur when once meltingthe raw material mixture for forming a RE--Ba--Cu--O superconductor.Thus, it is necessary for uniformly dispersing the raw materials tocrush the raw materials into very fine powder. Moreover, the density ofa sample (or specimen), which is melted and recrystallized by using fineraw materials, becomes very high. Thus, the conventional method hasproblems in that the diffusion velocity of oxygen is low and that anoxygen annealing time is very long.

The present invention is accomplished to solve the aforementionedproblems of the prior art.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide an oxidesuperconductor which has superconducting properties and uniform highercritical current density.

Further, another object of the present invention is to provide a simplemethod of manufacturing such an oxide superconductor at a low cost.

To achieve the foregoing objects, in accordance with an aspect of thepresent invention, there is provided an oxide superconductor (hereundersometimes referred to as a first oxide superconductor of the presentinvention) in which fine particles (or grains) of a RE₂ BaCuO₅ phase (REis one or more kinds of rare earth elements including Y) are dispersedin a crystal of a REBa₂ Cu₃ O_(7-x) phase. In the first oxidesuperconductor of the present invention, fine voids, each of which has adiameter of 10 to 500 μm, are dispersed therein.

In accordance with another aspect of the present invention, there isprovided another oxide superconductor (hereunder sometimes referred toas a second oxide superconductor of the present invention) in which fineparticles of a RE₂ BaCuO₅ phase (RE is one or more kinds of rare earthelements including Y) are dispersed in a crystal of a REBa₂ Cu₃ O_(7-x)phase. In the second oxide superconductor of the present invention, thedensity thereof is 5 to 6 g/cm³.

In accordance with a further aspect of the present invention, there isprovided still another oxide superconductor (hereunder sometimesreferred to as a third oxide superconductor of the present invention) inwhich fine particles of a RE₂ BaCuO₅ phase (RE is one or more kinds ofrare earth elements including Y) are dispersed in a crystal of a REBa₂Cu₃ O_(7-x) phase. In the third oxide superconductor of the presentinvention, fine voids, each of which has a mean particle diameter (orsize) of 10 to 500 μm, are dispersed therein. Moreover, the densitythereof is 5 to 6 g/cm³.

In the case of an embodiment (hereunder sometimes referred to as afourth oxide superconductor of the present invention) of the first,second or third oxide superconductor of the present invention), thefourth oxide superconductor of the present invention contains 0.05 to 5in percent by weight (wt %) of one or more kinds of elements of metalsPt, Pd, Ru, Rh, Ir and Os and compounds thereof.

In the case of an embodiment (hereunder sometimes referred to as a fifthoxide superconductor of the present invention) of the first, second,third or fourth oxide superconductor of the present invention), thefifth oxide superconductor of the present invention contains 1 to 30 wt% of Ag.

In accordance with still another aspect of the present invention, thereis provided an oxide-superconductor manufacturing method (hereundersometimes referred to as a first method of the present invention) ofmanufacturing a RE--Ba--Cu--O oxide superconductor (RE is one or morekinds of rare earth elements including Y) by performing a treatment,which includes at least a burning process to be performed in a range oftemperatures that are higher than the melting point of a raw materialmixture containing a RE-compound raw material, Ba-compound raw materialand a Cu-compound raw material, on the aforesaid raw material mixture.The first method of the present invention further comprises the step ofcrushing the aforesaid raw material mixture into particles after theburning thereof, and of establishing the mean particle diameter (orsize) of one or all of the aforesaid raw materials as ranging from 50 to80 μm.

In the case of an embodiment (hereunder sometimes referred to as asecond method of the present invention) of the first method of thepresent invention, 0.05 to 5 wt % of one or more kinds of elements ofmetals Pt, Pd, Ru, Rh, Ir and Os and compounds thereof are added to theaforesaid raw material mixture.

In the case of an embodiment (hereunder sometimes referred to as a thirdmethod of the present invention) of the first or second method of thepresent invention, 1 to 30 wt % of Ag is further added to the aforesaidraw material mixture.

When manufacturing a RE--Ba--Cu--O oxide superconductor (RE is one ormore kinds of rare earth elements including Y) by performing atreatment, which includes at least a burning process to be performed ina range of temperatures that are higher than the melting point of a rawmaterial mixture containing a RE-compound raw material, Ba-compound rawmaterial and a Cu-compound raw material, on the aforesaid raw materialmixture, fine particles of the RE₂ BaCuO₅ phase, which have a meanparticle diameter of 1 to 30 μm or so, are dispersed in a crystal of theREBa₂ Cu₃ O_(7-x) phase. Consequently, the critical current can beincreased.

Moreover, according to the oxide superconductor manufacturing method ofthe present invention, the aforesaid raw material mixture is burned andpulverized into particles in such a manner that the mean particlediameter of one or all of the aforesaid raw materials is adjusted to asize in the range from 50 to 80 μm. Thus, the density of each compact(or pellet) is lowered to 4 to 5 g/cm³ by using such a raw materialmixture. When performing the melting and solidifying of the rawmaterials by using such low-density compacts, air or gas is left in thematerials because solidification commences from outer peripheralportions thereof. Thus, after the melting and crystallizing of thematerials, a large number of fine voids, whose diameters range from 10to 500 μm, are dispersed therein. The presence of such voids acceleratethe diffusion of oxygen in the materials, so that an annealing time canbe reduced. Consequently, an oxide superconductor, which exhibits auniform high critical current density, can be obtained. Incidentally, inthe portions whose depths are 2 to 5 mm or so (usually, 3 mm) from thesurface of the sample, the number of voids is small because the gas isdischarged from such portions to the outside of the sample even when thesample is solidified. However, the diffusion of oxygen is easilypromoted in such portions. Thus, the annealing time can be decreased,similarly as in the central portion of the sample. Consequently, anoxide superconductor, which exhibits a high critical current density,can be obtained. Moreover, the presence of such voids restrainsoccurrences of disordered microcracks that are not parallel to an a-bcrystallographic plane and become liable to occur when the outsidediameter and thickness of this material is larger than 40 mm and 20 mm,respectively. Incidentally, it is confirmed as a result of an experimentby the Inventors of the present invention that an oxide superconductorhaving similar advantageous effects can be obtained by setting the meanparticle diameter of one or more kinds of the aforementioned rawmaterials as ranging from 50 to 80 μm.

Incidentally, Pt sometimes gets mixed with raw materials from a platinumcrucible when making the raw material mixture for formingsuperconductors. However, the Inventors of the present invention haveconfirmed that similar advantages effects can be obtained even when 0.05to 5 wt % of Pt is contained in the raw material mixture and that anoxide superconductor having similar advantageous effects can be obtainedeven when 0.05 to 5 wt % of one or more kinds of elements of metals Pt,Pd, Ru, Rh, Ir and Os and compounds thereof are added to the rawmaterial mixture (incidentally, in the case of each of these compounds,the numerical values 0.05 to 5 wt % are those of a rate (expressed inpercent by weight) of the amount of such a metallic element contained inthe corresponding compound to the total amount of this compound).

Furthermore, when 1 to 30 wt % of metal Ag or of the compound powderthereof is added to the raw material mixture, the mechanical-strengthand water- resistance of the oxide superconductor are enhanced(incidentally, in the case of the compound powder of Ag, the numericalvalues 1 to 30 wt % are those of a rate (expressed in percent by weight)of the amount of Ag contained in the corresponding compound to the totalamount of this compound).

As above described, in accordance with the present invention, whenproducing a RE--Ba--Cu--O oxide superconductor, a raw material mixtureis pulverized, so that the mean particle diameter of at least one kindof raw materials contained in the raw material mixture ranges from 50 to80 μm. Thus, a large number of fine voids having diameters of 10 to 500μm are dispersed in the sample after the melting and crystallizingthereof. Consequently, the annealing time can be reduced. Moreover, anoxide superconductor, which exhibits a uniform high critical currentdensity, can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features, objects and advantages of the present invention willbecome apparent from the following description of preferred embodimentswith reference to the drawings in which like reference charactersdesignate like or corresponding parts throughout several views, and inwhich:

FIG. 1 is a scanning electron microscope photo showing a section of anoxide superconductive sample or specimen manufactured in the case of"Example 1" (to be described later) of the present invention;

FIG. 2 is a table showing a result of measurement of the criticalcurrent density (Jc) of each of four regions I, II, III and IV, whichare respectively obtained by partitioning the oxide superconductivesample manufactured in the case of the "Example 1" of the presentinvention every 3.5 mm from the top of the center axis thereof to thebottom, at a temperature of 77 K! in an external magnetic field 0 T!;

FIG. 3 is a table showing a result of measurement of the criticalcurrent density (Jc) of each of four regions I, II, III and IV, whichare respectively obtained by partitioning the oxide superconductivesample manufactured in the case of "Example 2" (to be described later)of the present invention every 3.5 mm from the top of the center axisthereof to the bottom,at a temperature of 77 K! in an external magneticfield 0 T!;

FIG. 4 is a table showing a result of measurement of the criticalcurrent density (Jc) of each of four regions I, II, III and IV, whichare respectively obtained by partitioning the oxide superconductivesample manufactured in the case of "Example 3" (to be described later)of the present invention every 3.5 mm from the top of the center axisthereof to the bottom, at a temperature of 77 K! in an external magneticfield 0 T!;

FIG. 5 is a table showing a result of measurement of the criticalcurrent density (Jc) of each of four regions I, II, III and IV, whichare respectively obtained by partitioning the oxide superconductivesample manufactured in the case of "Example 4" (to be described later)of the present invention every 3.5 mm from the top of the center axisthereof to the bottom, at a temperature of 77 K! in an external magneticfield 0 T!;

FIG. 6 is a table showing a result of measurement of the criticalcurrent density (Jc) of each of four regions I, II, III and IV, whichare respectively obtained by partitioning the oxide superconductivesample manufactured in the case of "Example 5" (to be described later)of the present invention every 3.5 mm from the top of the center axisthereof to the bottom, at a temperature of 77 K! in an external magneticfield 0 T!;

FIG. 7 is a scanning electron microscope photo showing a section of anoxide superconductive sample or specimen manufactured in the case of"Comparative Example 1" (to be described later) of the presentinvention;

FIG. 8 is a table showing a result of measurement of the criticalcurrent density (Jc) of each of four regions I, II, III and IV, whichare respectively obtained by partitioning the oxide superconductivesample manufactured in the case of the "Comparative Example 1" every 3.5mm from the top of the center axis thereof to the bottom, at atemperature of 77 K! in an external magnetic field 0 T!;

FIG. 9 is a table showing a result of measurement of the criticalcurrent density (Jc) of each of four regions I, II, III and IV, whichare respectively obtained by partitioning the oxide superconductivesample manufactured in the case of "Comparative Example 2" (to bedescribed later) every 3.5 mm from the top of the center axis thereof tothe bottom, at a temperature of 77 K! in an external magnetic field 0T!;

FIG. 10 is a graph illustrating a pattern of the change in temperaturein the melting and crystallizing of an oxide superconductive in the caseof "Example 6" (to be described later) of the present invention;

FIG. 11 is a table showing a result of measurement of the criticalcurrent density (Jc) of each of four regions I, II, III and IV, whichare respectively obtained by partitioning the oxide superconductivesample manufactured in the case of the "Example 6" of the presentinvention every 3.5 mm from the top of the center axis thereof to thebottom, at a temperature of 77 K! in an external magnetic field 0 T!;and

FIG. 12 is a table showing a result of measurement of the criticalcurrent density (Jc) of each of four regions I, II, III and IV, whichare respectively obtained by partitioning the oxide superconductivesample manufactured in the case of "Comparative Example 3" (to bedescribed later) every 3.5 mm from the top of the center axis thereof tothe bottom, at a temperature of 77 K! in an external magnetic field 0T!;

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the present invention will be described in detail withreference to the following examples.

EXAMPLE 1

In the case of this example, briefly, Y was used as RE of the REcompound composing a raw material mixture for forming an oxidesuperconductor. Further, such a raw material mixture was pulverized intopowder so that the mean particle diameter of the powder was about 60 μm.

First, after weighing powdery raw materials, namely, Y₂ O₃ powder, BaCO₃powder and CuO powder so that the composition ratio Y:Ba:Cu was18:24:34, only BaCO₃ powder and CuO powder were burned in a platinumcrucible at a temperature of 950 degrees centigrade for two hours. Thus,calcined powder containing BaCuO₂ and CuO was obtained (in a mole ratioof BaCuO2 to CuO, which was 24.10). Subsequently, this calcined powderwas pulverized by using a pot mill, so that the mean particle diameterwas about 2 μm. Then, the pulverized powder was mixed with Y₂ O₃ powderthat was preliminarily weighted and had a mean particle diameter ofabout 2 μm. Next, the temperature of this mixture powder was raised fromroom temperature to 940 degrees centigrade in the air for ten hoursFurther, the mixture powder was maintained at this temperature forthirty hours. Thereafter, the temperature of the mixture powder waslowered to room temperature and then, the mixture powder undergoes thecalcining. Further, this calcined mixture powder was pulverized in anagate mortar, so that the mean particle diameter of the mixture powderwas about 60 μm. Subsequently, this pulverized mixture powder waspress-molded into a disk-like compact which was 50 in outer diameter andwas 20 mm in thickness and was a raw material mixture for forming asuperconductor. At that time, a measurement of the density of thecompact was performed, so that the density thereof was 4.8 g/cm³.

Subsequently, this compact was put on an alumina board or substrate andwas then put into a semi-melted state by being heated to a temperatureof 1150 degrees centigrade in the air. Thereafter, the temperature ofthe compact was lowered to a temperature of 1000 degrees centigrade at arate of 10 degrees centigrade (°C.) per minute. Then, a preliminarilyproduced seed crystal of NdBaCuO molten material compound was broughtinto contact with an upper part of the compact in such a manner that thedirection of growth of the seed crystal was parallel with the c-axis.Further, a temperature gradient of 5° C./cm was vertically imposed ontothe compact in such a way that the upper part of the compact was at alower temperature side. Then, the compact was gradually cooled at a rateof 1° C./hr to a temperature of 900° C. Furthermore, the temperature ofthe compact was lowered at a rate of 1° C./hr to room temperature. Thus,the crystallization of the compact was performed.

Subsequently, the crystallized compact was placed in a furnace adaptedto be able to perform inert gas replacement. Then, the inner pressure ofthe furnace was reduced by a rotary pump to a pressure of 0.1 Torr.Thence, oxygen gas was poured into the furnace, so that the innerpressure of the furnace was equal to an atmospheric pressure and thatthe partial pressure of oxygen was 95% or more of the inner pressure ofthe furnace. Thereafter, the temperature in the furnace was raised fromroom temperature to a temperature of 600° C. over a time period of 10hours, during oxygen gas was simultaneously poured into the furnace at aflow rate of 0.5 L/min. Thereafter, a time period of 100 hours wasrequired to gradually lower the temperature in the furnace from 600° C.to 300° C. Thus, a sample of a superconductor was produced.

Then, the sample of the superconductor, which was obtained in theaforementioned manner, was cut. Further, when a section of the samplewas observed by using a scanning electron microscope, it was found thatfine particles of the Y₂ BaCuO₅ phase, which have particle diameters of0.1 to 30 μm, were dispersed in a crystal of the YBa₂ Cu₃ O₇₋ x phase,and that fine voids of diameters ranging from 10 to 500 μm weredispersed in a portion, whose depth was not less than about 3 mm, of thesample. Furthermore, when measured, the density of the entiresuperconductor sample was 5.6 g/cm3. Moreover, it was found that theentire sample was oriented in the direction of the c-axis and that thissuperconductor sample was substantially signal crystal. FIG. 1illustrates a scanning electron microscope photo taken at that time.

Further, the critical current density (Jc) of each of four regions I,II, III and IV, which were respectively obtained by partitioning theoxide superconductive sample every 3.5 mm from the top of the centeraxis thereof to the bottom, was measured at a temperature of 77 K! in anexternal magnetic field 0 T!. FIG. 2 shows values of the criticalcurrent density (Jc) in each of the four regions.

As above described, in the case of the oxide superconductor manufacturedby the manufacturing method of this embodiment, the fine voids wereuniformly dispersed therein. Thus, the diffusion rate of oxygen wasincreased. Thus, the entire sample has a high critical current densityby performing the annealing in a short time

EXAMPLE 2

In the case of this example, briefly, Yb was used as RE of the REcompound composing a raw material mixture for forming an oxidesuperconductor. Further, 0.5 wt % of each of the following kinds ofmetallic powder, namely, each of Pt powder, Pd powder, Ru powder, Rhpowder, Ir powder, Os powder and Re powder was added to and mixed with araw material mixture. Then, a resultant raw material mixture waspulverized into powder so that the mean particle diameter of the powderwas about 55 μm.

First, powdery raw materials, namely, Yb₂ O₃ powder, BaCO₃ powder andCuO powder were weighted so that the composition ratio Yb:Ba:Cu was22;26:36. Moreover, 0.5 wt % of each of the following kinds of metallicpowder, namely, each of Pt powder, Pd powder, Ru powder, Rh powder, Irpowder, Os powder and Re powder was added to and mixed with acorresponding mixture of such raw materials. Subsequently, thetemperature of these mixtures was raised from temperature to atemperature of 880° C. over a time period of 10 hours. Then, thesemixtures were maintained at a temperature of 880° C. for 30 hours.Thereafter, the temperature of the mixtures was lowered to roomtemperature over a time period of 10 hours. Then, the mixtures undergothe burning (or baking) Further, these calcined mixtures were pulverizedin an agate mortar, so that the mean particle diameter of the mixturepowder was about 55 μm. Subsequently, each of these pulverized mixtureswas press-molded into disk-like compacts, each of which was 50 mm inouter diameter and was 20 mm in thickness and was a raw material mixturefor forming a superconductor. At that time, the density of each of thecompacts was about 4.8 g/cm³.

Next, these compacts were put on an alumina board or substrate and werethen put into a semi-melted state by being heated to a temperature of1090 degrees centigrade in the air. Thereafter, the temperature of thecompacts was lowered to a temperature of 920 degrees centigrade at arate of 10 ° C./min. Then, a preliminarily produced seed crystal ofNdBaCuO molten material compound was brought into contact with an upperpart of each of the compacts in such a manner that the direction ofgrowth of the seed crystal was parallel with the c-axis. Further, atemperature gradient of 5° C./cm was vertically imposed onto each of thecompacts in such a way that the upper part of each of the compacts wasat a lower temperature side. Then, the temperature of the compact wasgradually lowered at a rate of 1° C. hr to a temperature of 850° C.Furthermore, the temperature of the compacts was lowered at a rate of 1°C. hr to room temperature. Thus, the crystallization of each of thecompacts was performed.

Subsequently, the crystallized compacts were placed in a furnace adaptedto be able to perform inert gas replacement. Then, the inner pressure ofthe furnace was reduced by a rotary pump to a pressure of 0.1 Torr.Thence, oxygen gas was poured into the furnace, so that the innerpressure of the furnace was equal to an atmospheric pressure and thatthe partial pressure of oxygen was 95% or more of the inner pressure ofthe furnace. Thereafter, the temperature in the furnace was raised fromroom temperature to a temperature of 600° C. over a time period of 10hours, during oxygen gas was simultaneously poured into the furnace at aflow rate of 0.5 L/min. Thereafter, a time period of 100 hours wasrequired to gradually lower the temperature in the furnace from 600° C.to 300° C. Thus, samples of a superconductor were produced.

Then, each of the samples of the superconductor, which was obtained inthe aforementioned manner, was cut. Further, when a section of each ofthe samples was observed by using a scanning electron microscope, it wasfound that fine particles of the Yb₂ BaCuO₅ phase, which have particlediameters of 0.1 to 30 μm, were dispersed in a crystal of the YbBa₂ Cu₃O₇₋ x phase, and that fine voids of diameters ranging from 10 to 500 μmwere dispersed in a portion, whose depth was not less than about 3 mm,of each of the samples in a ratio of 10 to 200 per mm2. Furthermore,when measured, the density of the entirety of each of the superconductorsamples was 5.8 g/cm³. Moreover, it was found that the entire sample wasoriented in the direction of the c-axis and that this superconductorsample was substantially signal crystal.

Further, the critical current density (Jc) of each of four regions I,II, III and IV that were respectively obtained by partitioning each ofthe disk-like oxide superconductive samples, to which Pt, Pd, Ru, Rh,Ir, Os and Re were respectively added, every 3.5 mm from the top of thecenter axis thereof to the bottom, was measured at a temperature of 77K! in an external magnetic field 0 T!. FIG. 3 shows values of thecritical current density (Jc) in the four regions of each of thesamples.

As above described, in the case of the oxide superconductor manufacturedby the manufacturing method of this embodiment, the fine voids wereuniformly dispersed therein. Thus, the diffusion rate of oxygen wasincreased. Consequently, the entire sample has a high critical currentdensity by performing the annealing in a short time.

EXAMPLE 3

In the case of this example, briefly, Ho was used as RE of the REcompound composing a raw material mixture for forming an oxidesuperconductor. Further, such a raw material mixture was pulverized intopowder so that the mean particle diameter of the powder was about 70 μm.

First, after powdery raw materials, namely, Ho₂ O₃ powder, BaCO₃ powderand CuO powder were weighted so that the composition ratio Ho:Ba:Cu was20:25:35, these raw materials were mixed with one another and furtherwere molten in a platinum crucible at a temperature of 1400 degreescentigrade for 30 minutes. Then, the molten materials were solidified byperforming the casting and quenching thereof. Subsequently, thissolidified or coagulated materials were pulverized into powder by usinga pot mill, so that the mean particle diameter of the powder was about 2μm. Next, the temperature of this mixture powder was raised from roomtemperature to 940 degrees centigrade in the air over a time period often hours, Further, the mixture powder was maintained at thistemperature for thirty hours. Thereafter, the temperature of the mixturepowder was lowered to room temperature over a time period of ten hours,and then, the mixture powder undergoes the calcining. Further, thiscalcined mixture powder was pulverized in an agate mortar, so that themean particle diameter of the mixture powder was about 70 μm.Subsequently, this pulverized mixture powder was press-molded into adisk-like compact which was 50 mm in outer diameter and was 20 mm inthickness and was a raw material mixture for forming a superconductor.At that time, a measurement of the density of the compact was performed,so that the density thereof was 4.7 g/cm³.

Subsequently, this compact was put on an alumina board or substrate andwas then put into a semi-melted state by being heated to a temperatureof 1150 degrees centigrade in the air. Thereafter, the temperature ofthe compact was lowered to a temperature of 1000 degrees centigrade at arate of 10 degrees centigrade (°C.) per minute. Then, a preliminarilyproduced seed crystal of NdBaCuO molten material compound was broughtinto contact with an upper part of the compact in such a manner that thedirection of growth of the seed crystal was parallel with the c-axis.Further, a temperature gradient of 5° C./cm was vertically imposed ontothe compact in such a way that the upper part of the compact was at alower temperature side. Thee, the temperature of the compact wasgradually lowered at a rate of 1° C./hr to a temperature of 900° C.Furthermore, the temperature of the compact was lowered at a rate of 10°C./hr to room temperature. Thus, the crystallization of the compact wasperformed.

Subsequently, the crystallized compact was placed in a furnace adaptedto be able to perform inert gas replacement. Then, the inner pressure ofthe furnace was reduced by a rotary pump to a pressure of 0.1 Torr.Thence, oxygen gas was poured into the furnace, so that the innerpressure of the furnace was equal to an atmospheric pressure and thatthe partial pressure of oxygen was 95% or more of the inner pressure ofthe furnace. Thereafter, the temperature in the furnace was raised fromroom temperature to a temperature of 600° C. over a time period of 10hours, during oxygen gas was simultaneously poured into the furnace at aflow rate of 0.5 L/min. Thereafter, a time period of 100 hours wasrequired to gradually lower the temperature in the furnace from 600° C.to 300° C. Thus, a sample of a superconductor was produced.

Then, the sample of the superconductor, which was obtained in theaforementioned manner, was cut. Further, when a section of the samplewas observed by using a scanning electron microscope, it was found thatfine particles of the Ho₂ BaCuO₅ phase, which have particle diameters of0.1 to 30 μm, were dispersed in a crystal of the HoBa₂ Cu₃ O₇₋ x phase,and that fine voids of diameters ranging from 10 to 500μm were dispersedin a portion, whose depth was not less than about 3 mm, of the sample.Furthermore, when measured, the density of the entire superconductorsample was 5.4 g/cm³. Moreover, it was found that the entire sample wasoriented in the direction of the c-axis and that this superconductorsample was substantially signal crystal.

Further, the critical current density (Jc) of each of four regions I,II, III and IV, which were respectively obtained by partitioning thisoxide superconductive sample every 3.5 mm from the top of the centeraxis thereof to the bottom, was measured at a temperature of 77 K! in anexternal magnetic field 0 T!. FIG. 4 shows values of the criticalcurrent density (Jc) in each of the four regions.

As above described, in the case of the oxide superconductor manufacturedby the manufacturing method of this embodiment, the fine voids wereuniformly dispersed therein. Thus, the diffusion rate of oxygen wasincreased. Consequently, the entire sample has a high critical currentdensity by performing the annealing in a short time.

EXAMPLE 4

In the case of this example, briefly, Nd was used as RE of the REcompound composing a raw material mixture for forming an oxidesuperconductor. Further, the aforesaid raw material mixture waspulverized into powder so that the mean particle diameter of the powderwas about 60 μm.

First, after weighing powdery raw materials, namely, Nd₂ O₃ powder,BaCO₃ powder and CuO powder so that the composition ratio Nd:Ba:Cu was1.8:2.4:3.4, only BaCO₃ powder and CuO powder were burned in a platinumcrucible at a temperature of 880 degrees centigrade for two hours. Thus,calcined powder containing BaCuO₂ and CuO was obtained (in a mole ratioof BaCuO₂ to CuO, which was 2.4:1.0). Subsequently, this calcined powderwas pulverized in an agate mortar, so that the mean particle diameterwas about 60 μm. Then, the pulverized powder was mixed with Nd₂ O₃powder, which was preliminarily weighted, and with 0.5 wt % of Ptpowder. Subsequently, this pulverized mixture powder was press-moldedinto a disk-like compact which was 50 mm in outer diameter and was 20 mmin thickness and was a raw material mixture for forming asuperconductor. At that time, a measurement of the density of thecompact was performed, so that the density thereof was 4.8 g/cm³.

Next, this compact was put on an alumina board or substrate and was thenput into a semi-melted state by being heated to a temperature of 1130degrees centigrade under the partial pressure of oxygen of 10-3 atm.Thereafter, the temperature of the compact was lowered to a temperatureof 1080 degrees centigrade at a rate of 10° C./min. Then, a crystalpreliminarily, which was produced in such a manner that particles of Nd₂Ba₀.5 Sr₀.5 CuO₅ phase was dispersed in a part of Nd(Ba₀.5 Sr₀.5)₂ Cu₃O₇₋ x phase in a composition ratio of the former to the latter 32 1:0.4and that the direction of growth thereof was parallel to the c-axis, wasused as a seed crystal and was brought into contact with an upper partof the compact. Further, a temperature gradient of 5° C./cm wasvertically imposed onto the compact in such a way that the upper part ofthe compact was at a lower temperature side. Then, the temperature ofthe compact was gradually lowered at a rate of 1° C./hr to roomtemperature. Thus, the crystallization of the compact was performed.

Subsequently, the crystallized compact was placed in a furnace adaptedto be able to perform inert gas replacement. Then, the inner pressure ofthe furnace was reduced by a rotary pump to a pressure of 0.1 Torr.Thence, oxygen gas was poured into the furnace, so that the innerpressure of the furnace was equal to an atmospheric pressure and thatthe partial pressure of oxygen was 95% or more of the inner pressure ofthe furnace. Thereafter, the temperature in the furnace was raised fromroom temperature to a temperature of 600° C. over a time period of 10hours, during oxygen gas was simultaneously poured into the furnace at aflow rate of 0.5 L/min. Thereafter, a time period of 100 hours wasrequired to gradually cool the furnace by lowering the temperature from600° C. to 300° C. Thus, a sample of a superconductor was produced.

Then, the sample of the superconductor, which was obtained in theaforementioned manner, was cut. Further, when a section of the samplewas observed by using a scanning electron microscope, it was found thatfine particles of the Nd₄ Ba₂ Cu₂ O₁₀ phase, which have particlediameters of 0.1 to 30 μm, were dispersed in a crystal of the NdBa₂ Cu₃O₇₋ x phase, and that fine voids of diameters ranging from 10 to 500 μmwere dispersed in a ratio of 10 to 200 per mm² in a portion, whose depthwas not less than about 3 mm, of the sample. Furthermore, when measured,the density of the entire superconductor sample was 5.9 g/cm³. Moreover,it was found that the entire sample was oriented in the direction of thec-axis and that this superconductor sample was substantially signalcrystal.

Further, the critical current density (Jc) of each of four regions I,II, III and IV, which were respectively obtained by partitioning theoxide superconductive sample every 3.5 mm from the top of the centeraxis thereof to the bottom, was measured at a temperature of 77 K! in anexternal magnetic field 0 T!. FIG. 5 shows values of the criticalcurrent density (Jc) in each of the four regions in which themeasurement was performed.

As above described, in the case of the oxide superconductor manufacturedby the manufacturing method of this embodiment, the fine voids wereuniformly dispersed therein. Thus, the diffusion rate of oxygen wasincreased. Thus, the entire sample has a high critical current densityby performing the annealing in a short time.

EXAMPLE 5

In the case of this example, briefly, Sm was used as RE of the REcompound composing a raw material mixture for forming an oxidesuperconductor. Further, the aforesaid raw material mixture waspulverized into powder so that the mean particle diameter of the powderwas about 60 μm. Further, 0.5 wt % of Pt powder and 10 wt % of Ag powderwere added thereto.

First, after weighing powdery raw materials, namely, Sm₂ O₃ powder,BaCO₃ powder and CuO powder so that the composition ratio Sm:Ba:Cu was1.8:2.4:3.4, only BaCO₃ powder and CuO powder were burned in a platinumcrucible at a temperature of 880 degrees centigrade for two hours. Thus,calcined powder containing BaCuO₂ and CuO was obtained (in a mole ratioof BaCuO₂ to CuO, which was 2.4:1.0). Subsequently, this calcined powderwas pulverized in an agate mortar, so that the mean particle diameterwas about 60 μm. Then, the pulverized powder was mixed with Sm₂ O₃powder, which was preliminarily weighted, and with 0.5 wt % of Pt powderand 10 wt % of Ag powder. Subsequently, this pulverized mixture powderwas press-molded into a disk-like compact which was 50 mm in outerdiameter and was 20 mm in thickness and was a raw material mixture forforming a superconductor. At that time, a measurement of the density ofthe compact was performed, so that the density thereof was 4.8 g/cm³.

Next, this compact was put on an alumina board or substrate and was thenput into a semi-melted state by being heated to a temperature of 1150degrees centigrade in the air. Thereafter, the temperature of thecompact was lowered to a temperature of 1000 degrees centigrade at arate of 10° C./min. Then, a preliminarily produced seed crystal ofNdBaCuO molten material compound was brought into contact with an upperpart of the compact in such a manner that the direction of growth of theseed crystal was parallel with the c-axis. Further, a temperaturegradient of 5° C./cm was vertically imposed onto the compact in such away that the upper part of the compact was at a lower temperature side.Then, the compact was gradually cooled at a rate of 1° C./hr to atemperature of 900° C. Furthermore, the temperature of the compact waslowered at a rate of 1° C./hr to room temperature. Thus, thecrystallization of the compact was performed.

Subsequently, the crystallized compact was placed in a furnace adaptedto be able to perform inert gas replacement. Then, the inner pressure ofthe furnace was reduced by a rotary pump to a pressure of 0.1 Torr.Thence, oxygen gas was poured into the furnace, so that the innerpressure of the furnace was equal to an atmospheric pressure and thatthe partial pressure of oxygen was 95% or more of the inner pressure ofthe furnace. Thereafter, the temperature in the furnace was raised fromroom temperature to a temperature of 600° C. over a time period of 10hours, during oxygen gas was simultaneously poured into the furnace at aflow rate of 0.5 L/min. Thereafter, a time period of 100 hours wasrequired to gradually cool the furnace by lowering the temperature from600° C. to 300° C. Thus, a sample of a superconductor was produced.

Then, the sample of the superconductor, which was obtained in theaforementioned manner, was cut. Further, when a section of the samplewas observed by using a scanning electron microscope, it was found thatfine particles of the Sm₂ BaCuO₅ phase, which have particle diameters of0.1 to 30 μm, were dispersed in a crystal of the SmBa₂ Cu₃ O₇₋ x phase,and that fine voids of diameters ranging from 10 to 500 μm weredispersed in a ratio of 10 to 200 per mm² in a portion, whose depth wasnot less than about 3 mm, of the sample. Furthermore, when measured, thedensity of the entire superconductor sample was 5.9 g/cm³. Moreover, itwas found that the entire sample was oriented in the direction of thec-axis and that this superconductor sample was substantially signalcrystal.

Further, the critical current density (Jc) of each of four regions I,II, III and IV, which were respectively obtained by partitioning theoxide superconductive sample every 3.5 mm from the top of the centeraxis thereof to the bottom, was measured at a temperature of 77 K! in anexternal magnetic field 0 T!. FIG. 6 shows values of the criticalcurrent density (Jc) in each of the four regions in which themeasurement was performed.

As above described, in the case of the oxide superconductor manufacturedby the manufacturing method of this embodiment, the fine voids wereuniformly dispersed therein. Thus, the diffusion rate of oxygen wasincreased. Thus, the entire sample has a high critical current densityby performing the annealing in a short time.

EXAMPLE 6

First, after weighing powdery raw materials, namely, Y₂ O₃ powder, BaCO₃powder and CuO powder so that the composition ratio Y:Ba:Cu was18:24:34, only BaCO₃ powder and CuO powder were burned in a platinumcrucible at a temperature of 880 degrees centigrade for 30 hours. Thus,calcined powder containing BaCuO₂ and CuO was obtained (in a mole ratioof BaCuO₂ to CuO, which was 24:10). Subsequently, this calcined powderwas pulverized by using a pot mill, so that the mean particle diameterwas about 2 μm. Then, the pulverized powder was mixed with Y₂ O₃ powder,which was preliminarily weighted and had a mean particle diameter ofabout 1 μm, and with 0.5 wt % of Pt powder, which was added and had amean particle diameter of about 0.02 μm. Next, the temperature of thismixture powder was raised from room temperature to 930 degreescentigrade in the air for ten hours. Further, the mixture powder wasmaintained at this temperature for thirty hours. Thereafter, thetemperature of the mixture powder was lowered to room temperature andthen, the mixture powder undergoes the calcining. Further, this calcinedmixture powder was pulverized in an agate mortar, so that the meanparticle diameter of the mixture powder was about 10 μm. Subsequently,this pulverized mixture powder was press-molded into a disk-like compactwhich was 50 in outer diameter and was 20 mm in thickness and was a rawmaterial mixture for forming a superconductor.

Subsequently, this compact was put on an alumina board or substrate andwas then put into a semi-melted state by being heated to a temperatureof 1130 degrees centigrade in the air. Thereafter, the temperature ofthe compact was rapidly lowered to a temperature of 1000 degreescentigrade (at a rate of 10° C./min) so that a temperature gradient of5° C./cm was vertically imposed onto the compact and that the upper partof the compact was at a lower temperature side. Then, a preliminarilyproduced seed crystal of NdBaCuO molten material compound was broughtinto contact with an upper part of the compact in such a manner that thedirection of growth of the seed crystal was parallel with the c-axis.Then, the compact was maintained nearly at a temperature of 995° C., atwhich the crystallization was commenced, nearly for 60 hours.Subsequently, the compact was gradually cooled at a rate of 1° C./hr toa temperature of 900° C. Furthermore, the temperature of the compact waslowered at a rate of 10° C./hr to room temperature. Thus, thecrystallization of the compact was performed. FIG. 10 illustrates aburning temperature pattern.

Subsequently, the crystallized compact was placed in a furnace adaptedto be able to perform inert gas replacement. Then, the inner pressure ofthe furnace was reduced by a rotary pump to a pressure of 0.1 Torr.Thence, oxygen gas was poured into the furnace, so that the innerpressure of the furnace was equal to an atmospheric pressure and thatthe partial pressure of oxygen was 95% or more of the inner pressure ofthe furnace. Thereafter, the temperature in the furnace was raised fromroom temperature to a temperature of 450° C. over a time period of 10hours, during oxygen gas was simultaneously poured into the furnace at aflow rate of 0.5 L/min. Thereafter, a time period of 200 hours wasrequired to gradually lower the temperature in the furnace from 250° C.to room temperature. Thus, a resultant material (namely, a sample of asuperconductor) was produced.

Then, the resultant material was cut. Further, when a section of thismaterial was observed by using a scanning electron microscope, it wasfound that fine particles of the Y₂ BaCuO₅ phase, which have particlediameters of 0.1 to 30 μm, were dispersed in a crystal of the YBa₂ Cu₃O₇₋ x phase, and that fine voids of diameters ranging from 10 to 500 μmwere dispersed in a ratio of 10 to 200 per mm² in a portion, whose depthwas not less than about 3 mm, of the material. Furthermore, there wasobtained the entire material that reflected the seed crystal and thedirection of growth thereof was oriented in the direction of the c-axisand that this material (namely, the superconductor sample) wassubstantially signal crystal. FIG. 1 illustrates a scanning electronmicroscope photo taken at that time.

Further, the critical current density (Jc) of each of four regions I,II, III and IV, which were respectively obtained by partitioning thedisk-like sample every 3.5 mm from the top of the center axis thereof tothe bottom, was measured at a temperature of 77 K! in an externalmagnetic field 0 T!. FIG. 11 shows values of the critical currentdensity (Jc) obtained as results of the measurement performed in each ofthe four regions.

As above described, in the case of the oxide superconductor manufacturedby the manufacturing method of this embodiment, the fine voids wereuniformly dispersed therein. Thus, the diffusion of oxygen wascompletely achieved. Consequently, the entire sample has a high criticalcurrent density by performing the annealing in a short time.

Comparative Example 1

Next, a superconductor sample, in which Y was used as RE of the REcompound composing a raw material mixture for forming an oxidesuperconductor and such a raw material mixture was pulverized intopowder so that the mean particle diameter of the powder was about 2 μm,will be described hereinbelow as a "comparative example 1".

First, after weighing powdery raw materials, namely, Y₂ O₃ powder, BaCO₃powder and CuO powder so that the composition ratio Y:Ba:Cu was18:24:34, these raw materials were mixed with one another and furtherwere molten in a platinum crucible at a temperature of 1400 degreescentigrade for 30 minutes. Then, the molten materials were solidified byperforming the casting and quenching thereof. Subsequently, thissolidified or coagulated materials were pulverized into powder by usinga pot mill, so that the mean particle diameter of the powder was about 2μm. Then, this pulverized mixture powder was press-molded into adisk-like compact which was 50 mm in outer diameter and was 20 mm inthickness and was a raw material mixture for forming a superconductor.At that time, a measurement of the density of the compact was performed,so that the density thereof was 5.2 g/cm³.

Subsequently, this compact was put on an alumina board or substrate andwas then put into a semi-melted state by being heated to a temperatureof 1150 degrees centigrade in the air. Thereafter, the temperature ofthe compact was lowered to a temperature of 1000 degrees centigrade at arate of 10 ° C./min. Then, a preliminarily produced seed crystal ofNdBaCuO molten material compound was brought into contact with an upperpart of the compact in such a manner that the direction of growth of theseed crystal was parallel with the c-axis. Further, a temperaturegradient of 5° C./cm was vertically imposed onto the compact in such away that the upper part of the compact was at a lower temperature side.Then, the temperature of the compact was gradually lowered at a rate of1° C./hr to a temperature of 900° C. Furthermore, the temperature of thecompact was lowered at a rate of 10° C./hr to room temperature. Thus,the crystallization of the compact was performed.

Subsequently, the crystallized compact was placed in a furnace adaptedto be able to perform inert gas replacement. Then, the inner pressure ofthe furnace was reduced by a rotary pump to a pressure of 0.1 Torr.Thence, oxygen gas was poured into the furnace, so that the innerpressure of the furnace was equal to an atmospheric pressure and thatthe partial pressure of oxygen was 95% or more of the inner pressure ofthe furnace. Thereafter, the temperature in the furnace was raised fromroom temperature to a temperature of 600° C. over a time period of 10hours, during oxygen gas was simultaneously poured into the furnace at aflow rate of 0.5 L/min. Thereafter, a time period of 100 hours wasrequired to gradually lower the temperature in the furnace from 600° C.to 300° C. Thus, a sample of a superconductor was produced.

Then, the sample of the superconductor, which was obtained in theaforementioned manner, was cut. Further, when a section of the samplewas observed by using a scanning electron microscope, it was found thatfine particles of the Y₂ BaCuO₅ phase, which have particle diameters of0.1 to 30 μm, were dispersed in a crystal of the YBa₂ Cu₃ O₇₋ x phase,and that fine voids of diameters ranging from 10 to 500 μm were sparselydispersed in a ratio of 0 to 9 per mm² in a portion, whose depth was notless than about 3 mm, of the sample. Furthermore, when measured, thedensity of the entire superconductor sample was 6.1 g/cm³. Moreover, itwas found that the entire sample was oriented in the direction of thec-axis and that this superconductor sample was substantially signalcrystal. However, several microcracks, which were neither found in thecase of the aforementioned "Example 1" to "Example 6" and were norparallel to the a-b crystallographic plane, occurred in thissuperconductor sample. FIG. 7 illustrates a scanning electron microscopephoto taken at that time.

Further, the critical current density (Jc) of each of four regions I,II, III and IV, which were respectively obtained by partitioning thisoxide superconductive sample every 3.5 mm from the top of the centeraxis thereof to the bottom, was measured at a temperature of 77 K! in anexternal magnetic field 0 T!. FIG. 8 shows values of the criticalcurrent density (Jc) in each of the four regions.

As above described, in the case of the oxide superconductor manufacturedby the manufacturing method of this "Comparative Example 1", there werefew fine voids in the sample. Thus, the diffusion rate of oxygen waslow. Consequently, in the case of performing the annealing in a shorttime, the critical current density in the vicinity of the centralportion of the sample was low.

Comparative Example 2

Next, a superconductor sample, in which Sm was used as RE of the REcompound composing a raw material mixture for forming an oxidesuperconductor and such a raw material mixture was pulverized intopowder so that the mean particle diameter of the powder was about 2 μm,will be described hereinbelow as a "comparative example 2".

First, after weighing powdery raw materials, namely, Sm₂ O₃ powder,BaCO₃ powder and CuO powder so that the composition ratio Sm:Ba:Cu was1.8:2.4:3.4, only BaCO₃ powder and CuO powder were burned in a platinumcrucible at a temperature of 880 degrees centigrade for two hours. Thus,calcined powder containing BauCO₂ and CuO was obtained (in a mole ratioof BaCuO₂ to CuO, which was 2.4:1.0). Subsequently, this calcined powderwas pulverized in an agate mortar, so that the mean particle diameter ofthe mixture powder was about 3 μm. Then, the pulverized powder was mixedwith Sm₂ O₃ powder, which was preliminarily weighted, and with 0.5 wt %of Pt powder. Subsequently, this pulverized mixture powder waspress-molded into a disk-like compact which was 50 mm in outer diameterand was 20 mm in thickness and was a raw material mixture for forming asuperconductor. At that time, a measurement of the density of thecompact was performed, so that the density thereof was 5.2 g/cm³.

Next, this compact was put on an alumina board or substrate and was thenput into a semi-melted state by being heated to a temperature of 1150degrees centigrade in the air. Thereafter, the temperature of thecompact was lowered to a temperature of 1000 degrees centigrade at arate of 10° C./min. Then, a preliminarily produced seed crystal ofNdBaCuO molten material compound was brought into contact with an upperpart of the compact in such a manner that the direction of growth of theseed crystal was parallel with the c-axis. Further, a temperaturegradient of 5° C./cm was vertically imposed onto the compact in such away that the upper part of the compact was at a lower temperature side.Then, the compact was gradually cooled at a rate of 1° C./hr to atemperature of 900° C. Furthermore, the temperature of the compact waslowered at a rate of 10° C./hr to room temperature. Thus, thecrystallization of the compact was performed.

Subsequently, the crystallized compact was placed in a furnace adaptedto be able to perform inert gas replacement. Then, the inner pressure ofthe furnace was reduced by a rotary pump to a pressure of 0.1 Torr.Thence, oxygen gas was poured into the furnace, so that the innerpressure of the furnace was equal to an atmospheric pressure and thatthe partial pressure of oxygen was 95% or more of the inner pressure ofthe furnace. Thereafter, the temperature in the furnace was raised fromroom temperature to a temperature of 600° C. over a time period of 10hours, during oxygen gas was simultaneously poured into the furnace at aflow rate of 0.5 L/min. Thereafter, a time period of 100 hours wasrequired to gradually cool the furnace by lowering the temperature from600° C. to 300° C. Thus, a sample of a superconductor was produced.

Then, the sample of the superconductor, which was obtained in theaforementioned manner, was cut. Further, when a section of the samplewas observed by using a scanning electron microscope, it was found thatfine particles of the Sm₂ BaCuO₅ phase, which have particle diameters of0.1 to 30 μm, were dispersed in a crystal of the SmBa₂ Cu₃ O₇₋ x phase,and that fine voids of diameters ranging from 10 to 500 μm were sparselydispersed in a ratio of 0 to 9 per mm2 in a portion, whose depth was notless than about 3 mm, of the sample. Furthermore, when measured, thedensity of the entire superconductor sample was 6.2 g/cm³. Moreover, itwas found that the entire sample was oriented in the direction of thec-axis and that this superconductor sample was substantially signalcrystal. However, several microcracks, which were neither found in thecase of the aforementioned "Example 1" to "Example 6" and were norparallel to the a-b crystallographic plane, occurred in thissuperconductor sample. FIG. 7 illustrates a scanning electron microscopephoto taken at that time.

Further, the critical current density (Jc) of each of four regions I,II, III and IV, which were respectively obtained by partitioning thisoxide superconductive sample every 3.5 mm from the top of the centeraxis thereof to the bottom, was measured at a temperature of 77 K! in anexternal magnetic field 0 T!. FIG. 9 shows values of the criticalcurrent density (Jc) in each of the four regions.

As above described, in the case of the oxide superconductor manufacturedby the manufacturing method of this "Comparative Example 2", there werefew fine voids in the sample. Thus, the diffusion rate of oxygen waslow. Consequently, in the case of performing the annealing in a shorttime, the critical current density in the vicinity of the centralportion of the sample was low.

Comparative Example 3

After weighing powdery raw materials, namely, Y₂ O₃ powder, BaCO₃ powderand CuO powder so that the composition ratio Y:Ba:Cu was 18:24:34, theseraw materials were mixed with one another and further were molten in aPt crucible at a temperature of 1400 degrees centigrade for 30 minutes.Then, the molten materials were solidified by performing the casting andquenching thereof. Subsequently, this solidified or coagulated materialswere pulverized into powder by using a pot mill, so that the meanparticle diameter of the powder was about 2 μm. Then, this pulverizedmixture powder was press-molded into a disk-like compact which was 50 mmin outer diameter and was 20 mm in thickness and was a raw materialmixture for forming a superconductor.

Subsequently, this compact was put on an alumina board or substrate andwas then put into a semi-melted state by being heated to a temperatureof 1130 degrees centigrade in the air. Thereafter, the temperature ofthe compact was rapidly lowered to a temperature of 1000 degreescentigrade at a rate of 10° C./min so that a temperature gradient of 5°C./cm was vertically imposed onto the compact and that the upper part ofthe compact was at a lower temperature side. Then, a preliminarilyproduced seed crystal of NdBaCuO molten material compound was broughtinto contact with an upper part of the compact in such a manner that thedirection of growth of the seed crystal was parallel with the c-axis.Then, the compact was maintained nearly at a temperature of 995° C., atwhich the crystallization was commenced, nearly for 60 hours.Subsequently, the compact was gradually cooled at a rate of 1° C./hr toa temperature of 900° C. Furthermore, the temperature of the compact waslowered at a rate of 10° C./hr to room temperature. Thus, thecrystallization of the compact was performed.

Subsequently, the crystallized compact was placed in a furnace adaptedto be able to perform inert gas replacement. Then, the inner pressure ofthe furnace was reduced by a rotary pump to a pressure of 0.1 Torr.Thence, oxygen gas was poured into the furnace, so that the innerpressure of the furnace was equal to an atmospheric pressure and thatthe partial pressure of oxygen was 95% or more of the inner pressure ofthe furnace. Thereafter, the temperature in the furnace was raised fromroom temperature to a temperature of 450° C. over a time period of 10hours, during oxygen gas was simultaneously poured into the furnace at aflow rate of 0.5 L/min. Thereafter, a time period of 200 hours wasrequired to gradually lower the temperature in the furnace from 250° C.to room temperature. Thus, a resultant material (namely, a sample of asuperconductor) was produced.

Then, the resultant material was cut. Further, when a section of thismaterial was observed by using a scanning electron microscope, it wasfound that fine particles of the Y₂ BaCuO₅ phase, which have particlediameters of 0.1 to 30 μm, were dispersed in a crystal of the YBa₂ Cu₃O₇₋ x phase. However, voids of diameters ranging from 10 to 500 μm weresparsely dispersed in a ratio of 0 to 9 per mm² in a portion, whosedepth was not less than about 3 mm, of the sample. Furthermore, it wasfound that the entire sample reflected the seed crystal and was orientedin the direction of the c-axis and that this superconductor sample wassubstantially signal crystal.

Further, the critical current density (Jc) of each of four regions I,II, III and IV, which were respectively obtained by partitioning thisoxide superconductive sample every 3.5 mm from the top of the centeraxis thereof to the bottom, was measured at a temperature of 77 K! in anexternal magnetic field 0 T!. FIG. 12 shows values of the criticalcurrent density (Jc) in each of the four regions.

As above described, in the case of the oxide superconductor manufacturedby the manufacturing method of this "Comparative Example 3", there werefew fine voids in the sample. Thus, the diffusion rate of oxygen waslow. Consequently, in the case of performing the annealing in a shorttime, the critical current density in the vicinity of the centralportion of the sample was low.

Incidentally, in the foregoing description, there have been describedthe examples that respectively employ Y, Yb, Ho, Nd and Sm as RE.However, it was confirmed that oxide superconductors, which weremanufactured by using other rare earth elements as RE and performing thesame methods as used in the cases of the aforementioned examples, alsohad uniform high critical current densities.

Further, in the foregoing description, there have been described theexamples that respectively use metallic Pt powder, Pd powder, Ru powder,Rh powder, Ir powder, Os powder and Re powder. However, it was confirmedthat oxide superconductors, to which 0.05 to 5 wt % of compoundscontaining such elements were added, also had uniform high criticalcurrent densities.

Furthermore, in the case of the aforementioned examples, after the seedcrystal was brought into contact with the compact, the compact wascooled at a rate of 1° C./hr to a temperature of 900° C. It wasconfirmed that even when the compact was maintained at a halfwaytemperature in this cooling process, the crystal grew larger and themagnetic properties were enhanced.

Although the preferred embodiments of the present invention have beendescribed above, it should be understood that the present invention isnot limited thereto and that other modifications will be apparent tothose skilled in the art without departing from the spirit of theinvention.

The scope of the present invention, therefore, is to be determinedsolely by the appended claims.

What is claimed is:
 1. An oxide superconductor comprising:fine particlesof a RE₂ BaCuO₅ phase (RE is one or more kinds of rare earth elementsincluding Y); and crystals of a REBa₂ CU₃ O₇₋ x phase, wherein said fineparticles of the RE₂ BaCuO₅ phase are dispersed in each of said crystalsof a REBa₂ Cu₃ O₇₋ x phase, wherein fine voids, whose mean diameterranges from 10 to 500 μm, are dispersed therein, and wherein a densitythereof is 5 to 6 g/cm³.
 2. The oxide superconductor according to claim1, wherein said oxide superconductor contains 0.05 to 5 in percent byweight (wt %) of one or more kinds of elements of metals Pt, Pd, Ru, Rh,Ir and Os and compounds thereof.
 3. The oxide superconductor accordingto claim 1, wherein said oxide superconductor of the present inventioncontains 1 to 30 wt % of Ag.
 4. An oxide superconductor manufacturingmethod of manufacturing the RE--Ba--Cu--O oxide superconductor of claim1 by a treatment, which includes at least a burning step to be performedin a range of temperatures that are higher than a melting point of a rawmaterial mixture containing a RE-compound raw material, Ba-compound rawmaterial and a Cu-compound raw material, on said raw material mixture,said method further comprising:a step of crushing said raw materialmixture into particles after burning thereof, and of establishing themean particle diameter of one or all of said raw materials as rangingfrom 50 to 80 μm.
 5. The oxide superconductor manufacturing methodaccording to claim 4, which further comprises a step of adding 0.05 to 5wt % of one or more kinds of elements of metals Pt, Pd, Ru, Rh, Ir andOs and compounds thereof to said raw material mixture.
 6. The oxidesuperconductor manufacturing method according to claim 4, which furthercomprises a step of adding 30 wt % of Ag to said raw material mixture.